1. Field of the Invention
The present invention relates to an electrostatic potential measuring method for measuring a surface potential of a photosensitive drum used, for example, for a printer making use of an electrophotographic system or a copying machine.
2. Description of the Prior Art
Generally, in a printer or copying machine of this kind, a series of image forming processes are executed as described below. First, the surface of the photosensitive drum is uniformly charged in a dark place by a charging device. Next, an image portion of the charged surface of the photosensitive drum is illuminated by light through an exposing device, such as an LED array. At this time, a charge is removed from the image portion exposed to light (photoconductive phenomenon). An electrostatic latent image with a charge left thereon is formed on a portion other than the image portion not exposed to light of the photosensitive drum. Then, a toner (colored fine particles charged with the same polarity as the electrostatic latent image is supplied to the latent image by a developing unit to obtain a toner image (visible image). Further, a recording paper is overlaid on the toner image, and a charge with a polarity opposite to the charged polarity of the toner is applied to the recording paper by means of a transfer unit from the back of the recording paper. The toner image is then transferred to the recording paper by an electrostatic force. Heat or pressure is applied by a fuser to the transferred toner image and fused to the recording paper to provide a permanent image. On the other hand, the latent image charge on the photosensitive drum after transfer is discharged by a discharger (a discharging lamp). Any residual toner that is not transferred and remains on the photosensitive drum is removed by a cleaner.
The aforementioned latent image form is called a negative latent image, and in this case, the developing method is called a reversal developing method. On the other hand, in a copying machine or the like, a positive latent image is used as the latent image form, and in this case, a developing method called a normal developing method is used. In the positive latent image, portions other than the image portion are illuminated by an exposing device, conversely to the case of the negative latent image, to obtain an electrostatic latent image with a charge left on the image portion. In this case, a toner charged in polarity opposite to the electrostatic latent image is supplied to the latent image by the developing unit.
In the case where either the reversal developing method or the normal developing method is employed, attention is paid to the photosensitive drum. The electrostatic action, i.e., the charging or discharging action, is repeatedly carried out with respect to the surface of the drum. Here, to know the charging or discharging properties which comprise the characteristics of the photosensitive drum is very important, since formation of high quality image relies thereon. That is, if the charging or discharging properties which comprise the characteristics of the photosensitive drum are known, (in installation of devices which induce the electrostatic action such as a charger, a transfer unit or the like), it is possible to know the influence of the shape of the charger or the like on the operation thereof. It is further possible to determine an optimum installation place or remote distance with respect to the photosensitive drum, and it is possible to determine the optimum operating timing of the devices. Furthermore, for a photosensitive drum used for a long period of time, it is also possible to know the state of degradation of the photosensitive member being used. Generally, as a method for knowing the degree of the surface potential of the photosensitive drum, there is a potential measuring method using a surface potential meter. As the surface potential meter, various systems such as a radiation ionizing type, a DC amplifying type, an AC conversion type, etc. have been developed and used.
In individual machineries and devices such as a printer, a copying machine, etc., it is important for obtaining the optimum image formation to know the characteristics of photosensitive drums individually used. As for the example of a charging process, it is important for this process to charge the surface of the photosensitive drum quickly and uniformly to a predetermined potential. Accordingly, in this case, necessary information includes the optimum shape of a charger, the time required to reach a predetermined potential, and the distribution of potential on the photosensitive drum. Other necessary information includes attenuation in potential caused by exposure in the photosensitive drum, residual potential, charging record, durability, etc.
However, the value of the surface potential of the photosensitive drum measured by use of the surface potential meter is a mere average value obtained according to the number of measured samples. That is, since the photosensitive drum is rotating, the potential measured by the surface potential meter is merely obtained by sampling some of the instantaneous measured data at measuring points on the photosensitive drum and subjecting the resulting data to data processing. Consequently, according to such a measuring method as just mentioned, it is not possible to obtain information on whether or not the surface potential of the photosensitive drum reaches a predetermined potential after the passage of a predetermined time.
The present invention has been accomplished in view of the aforementioned circumstances. An object of the present invention is to provide an electrostatic potential measuring method capable of simply examining the optimum shape of a charger or the like, and various electrostatic characteristics of a photosensitive drum.